1. Field of the Invention
The present invention relates to a projection type image display apparatus which projects light modulated by a digital micromirror device (hereinafter referred to as DMD) on a display screen to display images.
2. Background Arts
There is a projection type image display apparatus which modulates illumination light from an illumination optical system with using a DMD to form an image light, which is then projected on a display screen through a projection optical system (see, for example, U.S. Pat. No. 6,588,908 corresponding to the Japanese patent laid-open publication No. 2002-350775). This projection type image display apparatus is of widespread use as a big screen TV receiver. The DMD is made up with a plurality of swingable mirror elements (hereinafter called mirror elements) arranged in a matrix, each of which elements can swing between an ON position to reflect the illumination light toward the projection optical system and an OFF position to reflect the illumination light off the projection optical system. Operating on modulated image signals, the DMD holds the mirror elements in the ON position either for a long duration of time to brighten the pixels or for a short duration of time to darken the pixels. Such control of the illumination light amount onto the display screen achieves proper light modulation that accords the image signals.
As shown in FIGS. 11A and 11B, the DMD 101 is composed of a substrate 103 and a cover 106 for covering a front face of the substrate 103. Mounted on the substrate 103 is the mirror unit 102, which has plural mirror elements arranged in a matrix. Around the mirror unit 102, bonding pads 104 are provided to connect the mirror unit 102 to a driving circuit through wires. If laid bare, the bonding pads 104 will spoil the appearance of the DMD, and even worse, they will diffusely reflect the illumination light to create harmful light (or noise light). The cover 106 is therefore divided into two portions, a light transmitting portion 107 that transmits the illumination light heading to an element face 102a of the mirror unit 102 and a light shielding portion 108 that surrounds the light transmitting portion 107 to block the illumination light heading off the element face 102a. The cover 106 is, for example, a transparent glass 106a with its rear face coated with a frame shaped light shielding layer 106b, and the uncoated portion thus forms the light transmitting portion 107. The light transmitting portion 107 is nearly a 4:3 aspect ratio rectangle that conforms to the shape of the element face 102a, so that the light shielding portion 108 can surely cover the substrate 103 except for the element face 102a. Such shielding contributes not only to improve the appearance of the DMD, but also to prevent the creation of the harmful light resulting from the bonding pads 104.
Even with the cover 106, however, if the illumination light runs off the light transmitting portion 107 to define the illumination area there, the illumination light reflects diffusely on a boundary of the light transmitting portion 107 and the light shielding portion 108, causing the harmful light. Therefore a mask plate with a substantially rectangular mask aperture, which conforms to the shape of the light transmitting portion 107, is disposed near a light exit section of the illumination optical system (see, for example, Japanese patent laid-open publication No. 8-227034 and No. 10-253923).
Each of the mirror elements, which constitute the element face 102a of the DMD 101, can swing on a rotation axis extending on the diagonal line of the mirror element. Considering the reflection efficiency of the mirror element taking such posture, a light axis LA of the illumination light striking the element face 102a should preferably be inclined, with respect to the normal N to the element face 102a, in a diagonal direction of the element face. Unfortunately, when the light axis LA is inclined, an illumination area 110 becomes a rhombus shape, not a rectangle shape, on the light receiving surface of the DMD 101. It is particularly difficult to form the illumination area 110 to fit within the light transmitting portion 107 and entirely cover the element face 102a at the same time. The creation of the harmful light is therefore hardly prevented on the boundary of the light transmitting portion 107 and light shielding portion 108.
In addition, if the illumination area 110 of such rhombus shape is stretched to the light shielding portion 108, the cover 106 is dimly illuminated by diffuse reflection of the illumination light. This results in unnecessary illumination in the peripheral area around the element face 102a of the mirror unit 102, glimmering the suppose-to-be dark areas of the projected image on the display screen.